Stella McCartney proposed that we take the tube. The concert was at the 02 arena in London’s East End, and the traffic that December evening was sure to be heavy. She tried to make the prospect of cold stations and uncertain routes seem an adventure, but her husband, Alasdhair Willis, a tall, pleasant man with a slight Yorkshire accent, prevailed upon her (“Stell . . .”) to order a car. So at 6:45, joined by their son Miller, 6, and daughter Bailey, 5, who were being allowed to stay up late on a school night, along with a nanny, we set off in a black minivan to see her 69-year-old father perform a three-hour extravaganza of Beatles and Wings favorites with Jumbotron flashbacks of the Fab Four. Two more children, Beckett, 3, and a baby girl named Reiley, stayed home with a baby sitter in Notting Hill.

As Miller and Bailey clowned and laughed on the jump seat, Stella called her father to report our progress. Miller, who had already announced, “I want Granddad to sing ‘Back in the U.S.S.R.’!” then got on the phone, his freckled face beaming as he made his request. Stella gave an amused sigh. She was wearing a dark gray sweater — a sample from a knitwear fitting held that afternoon — with jeans and black high heels. Despite the number of sweaters and leggings she and her design team had pinned, pulled and ripped apart, the four-hour session ended on schedule at 4 p.m., and despite how numbingly tedious this work had been, she seemed refreshed by the thought of going to watch Bailey’s ballet recital, which was scheduled for 5 at the nearby home of a friend. After that, of course, there was the concert.

In the arena’s backstage, Stella and her husband greeted several tour hands. The corridor was crowded with friends and other special guests spilling out of a party room where drinks were being served. Ron Wood of the Rolling Stones wandered past, trailed by a young woman. Paul McCartney’s new wife, Nancy Shevell, a slim, fine-boned woman who had been helping her husband get ready, came up, and Stella gave her a hug. We found Paul in his dressing room, a suite done in velvet-dark hues with a row of suits and shirts along one wall. He shook hands and then excused himself, saying he still had some warming up to do, and went into the next room, followed by Bailey and Miller. Soon there was the sound of gargling.

When the concert got going, Stella was on her feet, swaying and tossing her reddish blond hair and singing along. She has an excellent voice. She also inherited her father’s chubby-cheeked countenance and his self-armored ease. By contrast, Mary McCartney, Stella’s older sister and confidante, who came to the show with two of her sons and their young friends, seemed happy to remain seated most of the time. A photographer, Mary is darkly pretty and laid-back, but like her mother, Linda, she has no wish to master public life. Later, when I spoke to Laura Eastman Malcolm, Linda’s younger sister, she described a trip in the family’s station wagon: “Paul was driving, and Mary turns to me and in this disapproving tone says: ‘Do you like the fans? I’m sure you hate them!’ She was 8. Paul loves the fans. He can deal with them.” The implication is that Stella can also deal with public life. She says that, sure, the fans used to freak her out, too. “You’re like a little wall of defense,” she told me a few days later. “There’s dad, and you’re like this little wall in front, trying to spot who’s going to attack.” She laughed. “And then you grow up, and you realize he’s cool with it. He’s done it since he was 17.”

Stella wanted to stay for the final encore, but Alasdhair, in a quiet voice, said that it was getting late and maybe we should leave, and Stella, without dropping her eyes from his face, said all right. The ride back to Notting Hill went quickly, with Stella now between Bailey and Miller on the jump seat. The children showed no sign of sleepiness, and Miller begged for a story.

Knowing that her son was fascinated by World War II history, Stella, who had been reading a book of letters written by the legendary Mitford sisters, began telling the story of Unity Mitford, the daughter of an aristocratic British family who insinuated herself into Hitler’s inner circle and later shot herself in the head after Britain declared war on Germany. Suddenly, horribly, McCartney realized she couldn’t possibly finish the story. She giggled with loopy fatigue. “Oh, dear!” Miller looked up, waiting for the denouement. “And then,” she said hurriedly, “the English lady became very sad.”

The next morning, Stella would board a plane to Rome, where she was opening a new shop.

She may be one of the most interesting designers in the world today, but when she started out, the fashion world didn’t have many nice things to say about Stella McCartney. A prominent designer in New York, at the mention of her name, used to shriek like a blue jay, “The incredibly untalented Ms. McCartney!” And there was the public put-down by Karl Lagerfeld when, in 1997, he learned that he was being succeeded at Chloé by a 25-year-old woman who was only two years out of fashion school. “I think they should have taken a big name,” he snipped. “They did, but in music, not fashion.”

The man who hired McCartney for Chloé, Mounir Moufarrige, surely recognized the value of her famous family name. Moufarrige, an unctuous, hand-kissing executive, is the same individual who, a dozen years later, made Lindsay Lohan “artistic adviser” of Ungaro. McCartney was well aware of his game but also felt confident that she could run a Paris fashion house. After all, few major designers have come into the business better prepared than McCartney — if being exposed, from birth, to the demands of touring, celebrity and the media counts as preparation. As for Moufarrige, she said with a wry smile, “He was really my last connection to the McCartney thing.”

McCartney took with her to Chloé her best friend and classmate from Central Saint Martins, Phoebe Philo, with whom she had already started a talked-about label in her apartment in Notting Hill. “I mean, we were living in each other’s pockets,” McCartney says. “We were stuck like glue.” The clothes that she and Philo designed for Chloé were exactly what you’d expect from two English girls out to make their mark in a Paris dominated by the brash talents of John Galliano at Dior, Alexander McQueen and, as of October 2000, Tom Ford at Yves Saint Laurent. McCartney and Philo’s clothes were equally brash — freely and crudely expressing the desires of young women for sex and fun — with skintight pants, skimpy sequined dresses and airbrushed T-shirts (McCartney used T-shirts her parents bought in the ’70s as a source).

McCartney and Philo, both single at the time, were presumably interested in those things themselves. Gwyneth Paltrow, who has been a close friend of McCartney’s since they met in 1999, remembers being shocked at her wildness. “I’ve actually never seen someone go from such a free spirit to such a kind of beautifully conventional, hard-working life as a wife and mother,” she told me. “She was in full fun mode when I met her, and I just fell in love with her. She was shockingly honest. I always say there’s this kind of hidden ghetto side to Stella. She’s tough. She doesn’t back down from someone who might have less to lose than her.”

After McCartney left Chloé, some speculated that there was tension between her and Philo. But McCartney insists that “we didn’t have that fashion-chick falling out; we just moved on.” According to Ralph Toledano, the respected executive who took over Chloé from Moufarrige and today runs the fashion division of Puig, McCartney pulled the design weight at Chloé — not Philo, as many assumed. “Phoebe had a lot of input,” Toledano says, “but Stella was running the show.” One reason people concluded that McCartney was less talented was that Philo went on to triumph without her at Chloé ** — and later became the much celebrated minimalist designer of Céline — while McCartney’s first show for her own label was a flop. In what she now calls “a massive misstep,” McCartney sent out plastic guitar purses and clothes trashed with Cockney rhyming slang. The Allied bombing of Afghanistan had commenced the night before.

“I don’t think you can blame that!” she said, laughing at the memory. In a burst of British irreverence, she also hired Saint Martins students for her design team rather than professionals. “But the biggest mistake was I was trying to be something that I wasn’t.”

Where she has succeeded since then is in designing for exactly who she is. Now 40 years old, she has developed a witty, down-to-earth approach to dressing women like her: well-to-do, yes, but working women with lives of responsibility and complexity, women who have more use for a well-cut pantsuit for work or a roomy knit jumpsuit for hanging out with girlfriends than they do for a floor-length gown. McCartney is one of the very few designers, male or female, who make clothes for all hours of the day and not just the evening. In fact, red-carpet fashion has never been McCartney’s strength. Kate Hudson, a friend, recalled wearing a dress by McCartney to the Oscars and laughing with her afterward, because “I was on every worst-dressed list possible.” Much more successful was a group of ultracurvy black cocktail dresses with a sheer, polka-dot panel down one side that McCartney designed in early 2011, after the birth of her last child, as if to announce she was ready to go out again. They were a huge hit with customers; in magazines, the dress looked as good on Jane Fonda as it did on Gisele.

It sounds incredibly simple, but after more than a decade of dead-serious conceptualism, postmodern irony and Galliano-type showmanship, the fashion industry feels stuck for ideas: designing from life for life — rather than returning to the ’60s, say, or drawing inspiration from the “warrior woman” or some other female fantasy — feels fresh and modern.

The offices for McCartney’s company are on Golborne Road, not far from the pots-and-old-boots charm of Portobello Road, and a 10-minute drive from her home. There are three levels, with the 13 members of the design staff, including merchandisers and designers for shoes and children’s wear, on the top floor. Frederick Lukoff, the chief executive who previously worked at Apple and Lanvin, told me: “This is a company of mothers. It’s a unifying trait. These are organized, efficient moms. There are not many men, and the few are usually stunned by the level of organization of the women.”

In December, I attended several design meetings with McCartney and her team — Carolina Brodasca, Natasha Cagalj, Sara Jowett, Frances Howie — and I had the impression, most of the time, that they were finishing one another’s sentences. The people around McCartney seem to have no trouble voicing their opinions about designs. “I don’t think she’s competitive in that way,” Jowett explained. “She’s like, ‘Great, if you’re doing something brilliant, then let’s do that.’ ” As Tom Ford, a close friend of McCartney’s for the past 15 years, said: “Stella is not competitive in the girl arena. She is a girlfriend.”

I later asked McCartney about the lack of hierarchy and attitude in her office. “You know, the fashion industry has got some funny personalities,” she said delicately. “We don’t. We like a nice atmosphere. We don’t see the point in too much ego or competition. We just want to get on with it.”

When the Gucci Group (now PPR) agreed to back McCartney in 2001, the designer struck a very good deal for herself, the kind that was uncommon even then and almost certainly would not happen today. With the advice of her uncle, the lawyer John Eastman, McCartney negotiated equal ownership, 50-50. “They were fighting for 51 percent, and I just kept holding out,” she told me. “The very healthful part of the relationship is that we’re doing well, and they allow us to work in a fairly separate manner. Luckily, we’re a strong-enough brand, and a different-enough brand, so they can see the value in the 50 percent they don’t own.”

I asked McCartney if she ever considered asking her father for money to start her company so she could have full control. “We don’t do things like that in my family,” she said. “We work.” Plus, by teaming with Gucci, she would also get its production and distribution know-how.

When I spoke to François-Henri Pinault, the chairman of PPR, he recalled only two occasions of mild tension: once after he bought Puma in 2007 and asked McCartney if she would consider doing a line for Puma rather than for Adidas, where she has had a popular women’s performance line since 2004; and again when he transferred her much-liked chief executive to another PPR brand. She made her objections clear in both instances, Pinault said. (She prevailed on Adidas, but lost on the C.E.O.) He added: “It’s always the same with Stella. It’s a tough discussion, but when it’s over, it’s over. She knows that she’s the co-owner of the brand, its creative mind.”

Today the company has estimated annual sales of $120 million, a figure that doesn’t reflect the brand’s full retail power with licenses. And McCartney is expanding: in 2010, after a successful children’s-wear line for Gap, she started Stella McCartney Kids. And at this summer’s Olympics in London, British athletes will be wearing uniforms her company designed for Adidas, the team sponsor. This year, the company, which does most of its business in Europe and the United States, plans to open its first stores in China. McCartney has hinted in the past that she might not want to stay in fashion forever, but Lukoff dismisses the idea that she would ever sell. “That’s not the McCartney way,” the C.E.O. said, adding: “She’s building an asset. She looks at the figures with keen interest.”

Those figures have not been much affected by the recession. Her clothing is as expensive as other designer labels — a minidress from her summer line sells for about $2,200 — but because she offers a full wardrobe rather than primarily evening wear, there is a larger price range than in a lot of her competitors’ lines. (On the low end, for instance, is a T-shirt dress for $395.)

The sweater fitting on the day of the concert took place in a narrow, high-ceilinged studio at the rear of the building, with a view of a parking lot. Around 4 p.m., Brodasca told McCartney that she needed to look at some sunglasses designs before she left for her daughter’s ballet recital.

“The folder is big, so I might have it sent to your house, and you can look through it this week,” Brodasca said.

McCartney frowned. “They always do this,” she said to me. “They give me folders. ‘Take it home with you.’ When I go home, I don’t have time to do that.”

Brodasca continued, “We’re going to use them for the next two collections, so whatever you mark. . . .”

“I’ll do it now, then,” McCartney said.

She glanced at her BlackBerry. Each night, her longtime assistant, Samantha Merry, sends her an updated text of her appointments. That week, Beckett’s preschool was holding its Christmas pageant and Miller was in a concert. McCartney’s life is highly scheduled — as a matter of salvation, her sister Mary suggested. “I think Stella would crack up if she felt that she wasn’t organized and was missing things with the kids; that would make her unhappy.” She added, “And it’s not the way we grew up.”

After a quick look at the sunglasses, McCartney said it was time to leave for the recital, and I followed her out to her gray Mini Cooper. She tossed her black bag into the back next to two child’s seats. Leaving the lot, she hung a left, hit the clutch and shifted, and in a moment we were sailing through the dark, winding, residential streets of Notting Hill. A few minutes later, we rumbled to a stop in front of a large house and went inside to watch six little girls dance in a room seemingly created for such a purpose.

Later I asked McCartney if her Mini was turbocharged.

She laughed. After a moment she said: “My mum used to have a Mini Cooper. She had it custom-sprayed this metallic hot pink. She had a little microphone put in it, and she would sing to her eight-track. And she had a bench seat put in the front, and she’d always have four dogs in the back. My mum was renowned for collecting us late from school. I’d be on the village lane in Peasmarsh, and all of sudden — yeeooww — racing around the corner was this pink Mini with Neil Young screaming out.”

McCartney is in many ways her mother’s daughter. Like Linda, who died in 1998, McCartney doesn’t often wear makeup or fuss with her hair (though she doesn’t go so far as to cut it herself, as her mother did) or worry much about her critics. That so many of her designs seem to say, “This is me, whether you like it or not,” suggests the depth of her mother’s influence.

“She’s a paradox, I’ve always thought,” says Andrea Barron, a documentary-film producer, who has been a close friend of McCartney’s for more than 20 years. “She’s as male as she is female. She’s as feminine as she is strong. She’s always present. She’s one of those people, if you’re talking, who listens. She’s got balls, but she’s gentle. Every paradox, and I’ve thought this since Day 1.” For good or bad, contrast — the stylistic refuge of many designers in the past 20 years — is also at the heart of McCartney’s fashion, with a masculine pajama print complemented by a swirly white trim taken from a ceramic pattern, a combination she used recently for minidresses. But she does the effect well, in a way that feels true to her.

The seesawing between one thing and the other relates, naturally, to her upbringing: the public/private nature of Paul and Linda’s life, the to and fro between her “more gritty Liverpool side,” as she put it, and her cosmopolitan, well-to-do American side. In a work or social situation, you can almost see her drawing from both parts of herself, her conversational manner direct and mildly ironic. (One person, commenting on her ability to make people feel special, said, “I can never work out when it’s completely natural and when it isn’t.”)

Perhaps the most obvious way her parents have influenced her is in her thinking about animal cruelty. She is the only high-end designer who makes exclusively nonleather handbags and shoes. Her Falabella bags, which feature a chain detail along the edge, are hugely popular — according to Lukoff, accessory sales have grown fourfold in the last three years. Talking about her insistence on being leather-free, McCartney told me that she doesn’t think the rules of the fashion industry change very much. “They do on the design level, how a dress is made, but when it comes to how business is done, people pretty much follow the same rules,” she said. “Obviously I believe that using crocodile or leather to make a handbag is cruel. But it’s also not modern, you’re not pushing innovation.”

McCartney lobbies on behalf of PETA, as her parents did, appearing in a recent graphic video that exposes inhumane practices in the leather-goods industry. But she doesn’t hype her ethics with hangtags. In part that’s because she wants consumers to love a bag for its design, not as something eco, and in part it’s because the system of fair-trade, sustainable, ecological products is not perfect. “You can be making organic sweaters, fair trade, in Peru and the next month, that company is no longer in business,” she said. “Or it rained for a week, and the women couldn’t get to work. So there are a million things that can shift the rules, which I think is interesting.”

Some of McCartney’s best and most surprising designs seem to come from a world of happy homemakers. At the end of her spring 2011 show, following chic pantsuits and grown-up denim play outfits, she sent out shifts and blazers done in an oversize botanical print of lemons and oranges: a retro look with overtones of decorative aprons. (“A lot of people thought we had put a tablecloth on the runway,” Cagalj says.) But McCartney was comfortable with the association. “She’s two women, in a sense,” Barron told me. “She’s the woman who stands behind her man — I’m sure you’ve seen that with Alasdhair. She cooks. She’s that total ’50s woman. But she’s also the Madonna type, the woman who does everything herself.”

Her friends agree that marriage has changed McCartney. She and Willis met as she was setting up her company; he was then the publisher of Wallpaper magazine, which ran a creative agency on the side. He came to a meeting to discuss a logo design. Two years later, they married. “One of the smartest things she has ever done is choose Alasdhair as her husband and the father of her kids,” Paltrow told me. “They complement each other. He’s Northern, very practical — not buttoned-up, but he’s not like an American guy talking about his feelings all over the place. She’s like a firecracker.”

Their wedding was in a castle in Scotland: three days of games, teas and horseback riding, and a singular gesture of love by the groom. “We were having a cocktail on the lawn after the ceremony, and Alasdhair was making a toast, and he said, ‘I’d like to announce a new addition to our family.’ So we all thought she was pregnant,” Paltrow, a bridesmaid, recounted. “And then out from this row of trees comes this thoroughbred horse. Everyone burst into tears.” Later, the bride and groom — she having changed into a dress of her own design embroidered with stars — hit the dance floor in a bit of Fred and Ginger choreography that had been rehearsed for weeks. “It was insane,” Paltrow said.

Even if marriage has calmed her, she still doesn’t back down from a fight. The McCartney narrative has long included the story that, in 2001, when she left Chloé to start her own line, she turned down opportunities to design Givenchy and Gucci. According to McCartney, Tom Ford tried to persuade her — over her protests that she didn’t work with fur and leather — to take the creative-director job at Gucci while he designed Yves Saint Laurent, which Gucci Group had recently acquired. “He said: ‘Just come to my studio and look at everything. Maybe you’ll do it,’ ” she recalled, shaking her head. “As if all those exotic skins and corduroy hamster fur were going to turn me on and make me change my entire ethic.”

But when I spoke to Ford, he said: “It was never a conversation about taking over at Gucci. I think she might have interpreted that at a certain point.” He said he was aware of previous statements she made in the press, but “I just never corrected her.”

“Oh, he’s a lying, cheating . . . what?” McCartney exclaimed, when I repeated his comment. “That’s the weirdest thing. Why would he take me into an office and show me every dead animal? Oops!” She laughed and, frowning, said to me: “How are you going to handle that? ‘Stella says she got offered Gucci but she didn’t . . . LOSER!’ ” She continued to chuckle.

That Friday afternoon was a busy one for the design team on Golborne Road. In addition to garments for the fall runway show, set for March 5 in Paris, there was a preseason collection and a separate one that McCartney planned to show during London Fashion Week. Also, she and Brodasca had a Skype meeting with people from Adidas to discuss Olympic gymnastics uniforms. Starting at noon, at the table in the conference room, the design team began looking at line sheets and samples of fabric and yarn as they determined what they liked or still needed.

Looking at some jacquards and prints, McCartney said: “This is all very exciting, but it really needs monitoring. Because in my head, I’ve got 500 different shows here.”

Jowett said, “Well, the prints will hold it all together, and the technique will just add to the depth of what you — ”

McCartney shook her head. “I think we’ve just got to narrow it down.”

“We’re low on time and manpower, let’s put it that way,” Brodasca said.

“And what about our fluid woman, she’s not here,” McCartney said, using shorthand for a soft garment. “We have to help her.” She turned to Howie. “Almost a silk jersey. Is it not going to give you enough structure?”

When I left the office at 5, they were still working, empty teacups sitting amid the line sheets.

Almost every Friday night, McCartney and her husband leave London with their kids and dog and drive two hours to their country home, a former Georgian manor on about 400 acres, where they have a couple of horses, some ponies and a small flock of sheep, as well as extensive flower and vegetable gardens that Willis designed.

When I arrived at 9:30, McCartney, wearing jeans, an old brown sweater and heavy socks, was waiting in the doorway. She took my coat and said that she was making dinner. The children had already gone to bed. The house was utterly still. In the kitchen, Willis was throwing logs onto a fire in the walk-in fireplace, and McCartney was at the counter making pasta and a salad. A long trestle table stood in front of the fire. The walls were a pale blue, and the original stone floors were uneven and gleaming.

A large window above the sink overlooked the stable yard — part of the property still to be renovated, as I saw the next morning — and on the wall was a chart with each of the children’s names, followed by rows of stars designating rewards and privileges. “It’s a way of keeping order,” McCartney said. There was a corridor lined with boots and coats, and next to it was a breakfast room, where on one wall was an enormous photograph taken by Linda McCartney in the ’70s of Paul and his children playing by a wooden fence at their farm in Scotland. “We used to play on that fence for hours,” McCartney said.

McCartney had warned that spending time with her would require weathering some conditions: vomit, tantrums, dog hair. But her children were well behaved and seemingly secure. The next day, when Reiley went down for a nap, Beckett was off with his father, and Miller and Bailey had returned to the weekend’s central event — the arrival of four wild kittens that were now living in the barn — we sat on a big sofa in the living room and talked about Alexander McQueen, her good friend (they struck their deals with Gucci around the same time) who committed suicide in 2010. Before that, he was known to cut off contact for weeks at a time. She says she understands the desire to retreat.

“There are a lot of pressures,” she said. “It’s overwhelming to leave the house sometimes. Even if I didn’t have kids, I would be the same, but I think my brain is occupied with things other than fashion, fashion, fashion.”

Later, over a lunch McCartney prepared, I raised the idea with Willis that McCartney’s brand might one day extend beyond fashion, not unlike the examples of Ralph Lauren or Calvin Klein.

Willis, who owns a brand-consulting firm that represents David Beckham, among others, said: “I think Stella is associated with a value system — sustainability, moral values — that’s absolutely relevant to the times. The other brands are just beginning to catch on to her values, but they don’t have the authenticity. So the opportunity for her business to go into hotels and food — ”

“Eggplant Parmesan?” she interjected lightly.

Willis, serious, said to her, “Ultimately, what you offer the market is derived from how you live your life.” And then to me: “It’s not as if she’s fabricated these beliefs for the sake of her business. People see that.”

But McCartney was not interested in discussing her brand or her business or her future when the afternoon was slipping away and it would be dark soon.

“Do you want to stay in the kitchen or go for a walk?” she said, shifting her weight from one stockinged foot to the other. So we put on our coats and headed out into the field, taking Miller with us.